This invention relates in general to superconducting accelerator magnets and, more specifically to a method and apparatus for passively correcting for variations in magnetic field uniformity between different, though identically designed, magnets.
Magnetic fields guide particles, such as protons, through beam tubes. Particles can be accelerated to speeds approaching the speed of light by accelerators made up of a number of axially arranged high field magnets, with beam tubes under high vacuum that contain the particles.
In high energy physics research, such magnets have been used to accelerate and guide particles and cause collisions between them, revealing the presence of more fundamental particles and forces. Particle accelerators are also used in medical research and treatment, where tissues are bombarded with selected particles to change or destroy selected types of tissue, such as tumors. Other applications include x-ray lithography and protein crystallography.
Superconductors are materials, typically metals or ceramics, that lose all resistance when cooled below a critical temperature. Many materials have superconducting capabilities, although most only superconduct at temperatures approaching 0.degree. K. The most practical superconductors for use in superconducting magnets are those that superconduct at or above the boiling temperature of liquid helium; NbTi and Nb.sub.3 Sn are the most commonly used superconducting materials. Recently, ceramic superconductors, such as YBa.sub.2 Cu.sub.3 O.sub.7 have been developed that have critical temperatures above the boiling temperature of liquid nitrogen.
Magnets formed from superconductors and cooled below their critical temperatures are highly efficient and can provide extremely high magnetic fields. Such magnets are used in particle accelerators used in medical treatment, physics research and other fields. The Superconducting Supercollider will use thousands of superconducting magnets to guide particles through a very long, multi-magnet beam tube. These magnets require very high field uniformity in order to guide the particle beam through the beam tube without an excessive number of particles striking the inner surface of the tube and lost. The high field uniformity requirement in turn imposes very high tolerances for the parts and assembly of the magnets. Significant sources of error, in addition to assembly and random error stack-up, include the use of superconductors and other parts from different vendors. Magnetic non-uniformities or "multipoles" that exist in accelerator magnets have historically shown significant variations between different sets of magnets. Unless controlled, the large values of multipoles, as well as the magnet-to-magnet variability, can result in shorter accelerator operation times and hence increased accelerator costs. Greater control of the multipoles will allow broader manufacturing tolerances which will substantially lower the cost of accelerator magnet systems while improving performance. For example, collars are used to secure the magnet coils around the beam tube. It has been found that variations in the tightness of these collars from set to set of manufactured magnets are particularly significant in varying the persistent current and harmonic effects from set to set. In the past, attempts were made to use active corrector magnets positioned at selected locations in the system. These magnets, however, were not particularly effective, were difficult to fit into the system and significantly increased system cost. Similarly, attempting to enforce very tight tolerances is very expensive, often causing the rejection of a large percentage of completed magnets as out-of-tolerance.
Thus, there is a continuing need for methods of correcting variations in magnet multipoles between different sets of magnets.